The Physics of Creepy Crawlies and Ravenous Plants

For those attending the APS March Meeting in Boston, it might have seemed like Halloween came early this year. Scientists at the meeting presented work on the physics of a menagerie of things that go bump in the night, including the silk of spiders, the slither of snakes and the bite of a Venus flytrap.

The Amazing Spider-Materials
Spider silk is an amazing substance, stronger than steel and more stretchy than rubber. Scientists at the March Meeting reported on its electrical properties, which had never been previously investigated.

Eden Steven from the National High Magnetic Field Laboratory in Tallahassee found that spider silk can be used to make very small flexible wires. When nanoparticles of gold and carbon adhere to spider silk, they maintain their electrical conductivity, while at the same time the silk keeps its mechanical properties.

“To our surprise, gold really likes spider silk,” Steven said.

Tiny wires of gold on their own are rigid, which is not ideal for making wires. However gold- coated spider silk wires are flexible, stretchy and could be used to make flexible electronics.

Unlike silk from silkworms, spider silk is difficult to harvest. Spiders cannot be kept in close proximity as they are prone to attack each other. The only way to industrialize this application is to start synthesizing artificial spider silk, a long sought after Holy Grail of materials physics. Peggy Cebe of Tufts University has made a significant first step towards that goal by synthesizing polymers based on the silk of the golden orb weaver spider.

The polymers she and her team synthesized are very short; she described the length of the molecules they produced as “barely a polymer.” At its basic level, however, the molecules are the same. The longest molecules they have been able to produce are about 13,000 atomic mass units, while the polymers that make up spider silk are hundreds of thousands of amus.

“We’re trying to scale up to make longer molecules,” Cebe said. “We’re working towards that, but the synthesis … becomes more difficult.”

The polymers that the team has developed so far can be used to make microscopic hollow nodules that could be used for drug delivery.

Snakes on a Plane
Snakes are skilled climbers and researchers at the meeting unveiled a new aspect of their abilities. Hamidreza Marvi and his team from Georgia Institute of Technology found that snakes can toggle the scales on their belly between being grippy or slippy depending on whether they need to climb a tree or slide quickly across a surface.

“Snakes can actually change their frictional properties,” Marvi said. “Snakes can modify their scale’s angles of attack to change their frictional coefficients.”

There are several factors that affect the snake’s ability to slide around. Biologists had already identified tiny microstructures on the surface of each belly scale. The structures are directional, designed to grip the ground and prevent a snake from sliding backwards. Physicists found that there has to be another aspect that a snake can consciously control as well.

They put live snakes on slippery inclined planes and measured the angle at which the snake loses traction and slides down. The team first tried the experiment with a fully conscious snake, then again with one that had been knocked out with isoflurane. Sleeping snakes slid down at much lower angles than fully alert ones, showing there must be some way the snake is controlling its friction.

“When the snake is conscious, it can get a sense, feedback…and adjust accordingly,” Marvi said.

There is a ventral muscle that runs down the length of the snake’s belly that can make its scales stand on end. The researchers found that a change of just 5 degrees can affect the snake’s frictional coefficient by up to 50 percent. When climbing, a snake’s scales stand on end and dig into whatever surface it’s trying to scramble up. However when it needs to slip quickly across a plane, it pulls its scales parallel to its body to reduce friction.

Little Slap-Bracelet of Horrors
The chomp of a Venus flytrap is surely one of the most terrifying spectacles in the plant kingdom. New research into their infamous bite indicates that they might have a lot in common with popular toy jewelry from the 1980s.

“There could be these slap-bracelet type bi-stable structures embedded in the hinge,” said Zi Chen from the Washington University of Saint Louis. “We think the hinge is something that most people haven’t been paying attention to because most studies focus on the change of shape of the leaves.”

Slap-bracelets work because embedded in them, are thin sheets of stainless steel that act as springs. When the bracelets are unrolled, tension is built up in the spring, and it develops a slight negative curvature. When the bracelet is slapped against someone’s wrist, it releases the spring causing the bracelet to curl. Chen and his colleagues showed that the quick chomp of the flytrap comes from a similar spring-like structure that holds the leaves that make up the jaws of the flytrap.

“The hinge starts as a straight rod and then, as it grows and opens, the hinge slightly develops this negative curvature shape,” Chen said.

They tested this by taking high speed film of flytraps chomping down on insects, and observing how the hinge deformed. A flytrap is set off when small sensor hairs on the inside of its jaws are prodded twice by unsuspecting prey. The team was also able to get the trap to engage by poking the hinge with a needle, causing its spring to release and the trap to shut.

“It’s amazing to think that nature has figured out this complicated mechanism millions of years ago, to couple these dramatically different bi-stable behaviors in one species to function,” Chen said.